The present invention relates to computed tomography (CT) imaging apparatus; and more particularly, to the construction of detector arrays for multislice and volumetric CT ("VCT") systems.
In a current computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the "imaging plane." The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon a linear array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The x-ray source and detector array in a conventional third generation CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a "view" and a "scan" of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
A VCT scanner obtains in a single gantry rotation, 3D volume images that correspond to several conventional slice images. Several scanning approaches are used and they all employ a two-dimensional array of detector elements that gather attenuation measurements in the x, or "in-slice" direction and in the z, or "slice" direction. To reduce sampling aliasing artifacts in the reconstructed slice images, the resolution in both the slice and in-slice directions is maximized by reducing detector pitch. This is accomplished primarily by reducing the size of each detector element, but a limit is reached at which further resolution cannot be achieved because the active detector surface area becomes too small to produce an adequate signal. In other words, artifacts due to increased signal noise outweigh the reduction in image artifacts due to increased detector resolution.